In a radio communication network such as an Ultra Dense Network (UDN), a large number of communication nodes are deployed super densely. Hence, UDN nodes have to be of low cost and easy to be deployed. Synchronization is a leverage to simplify the design of network and device. The UDN nodes should be synchronized to meet complexity and cost control targets.
In addition, a typical deployment for UDN is in highly populated areas such as hot spots, office building, or downtown area at cities, where there are demands of high data rate service. Currently, data traffic is boosting rapidly while there is a clear bandwidth limit in low frequency bands. Hence, it is necessary for UDN to utilize a higher carrier frequency and a wider bandwidth in order to reach an even higher data rate. In order to have a high flexibility and low cost of deployment, wireless backhaul may be necessary for some access points (APs), but these UDN APs cannot obtain synchronization signal via wired networks. Furthermore, in order to reach high data rate, dynamic frequency division and allocation of radio resources are needed for neighboring UDN APs. In order to keep a frequency offset as small as possible between neighboring APs, an efficient time and frequency synchronization scheme is necessary. Therefore, it is desirable to provide an efficient synchronization method for the UDN.
Typically, the radio communication networks, such as Time Division Long Term Evolution (TD-LTE), Time Division-Synchronization Code Division Multiple Access (TD-SCDMA) and Code Division Multiple Access (CDMA) 2000, rely on Global Positioning System (GPS) to synchronize the whole network. In particular, each Base Station (BS) refers to a GPS reference timing signal to adjust its local timing. By doing so, the whole network can be synchronized to a satisfactory degree of accuracy in a sense that the remaining timing mismatch between neighboring BSs can be accommodated and solved by cyclic prefix of signals.
However, the existing method doesn't provide a straightforwardly valid means for the timing synchronization in the UDN. Adding GPS antenna and receiver to every UDN AP is not feasible for cost control reason. Also, most of UDN APs are deployed indoors. On the other hand, wireless backhaul requirement for some UDN APs rules out using wired line network to obtain synchronization reference.